Digital signal based apparatuses, such as mobile stations, mobile phones or the like, use high clock frequencies and frequencies of the digital buses. The frequencies have increased all the time. Due to the high frequency also the interference, i.e. radio interference, increases. This is mainly because the rising and falling times of the high frequency digital signals cannot be increased to the same extent than the rising and falling times of the digital signal when using lower frequencies. A known solution so far for the worst interference problems has been based on a frequency planning. Thus it has been pursued to find such clock frequencies and bus frequencies where there are no harmonics or mixing of frequencies at the applied radio channels. However more and more frequency ranges are to be used at the mobile phones. In addition the amount of different communication systems and accessories is increasing. Therefore a mobile phone can use and have systems such as GSM, WCDMA and CDMA each with various different frequency ranges. In addition the mobile phone could use and have WLAN, Bluetooth, GPS, Galileo, WUSB, FM radio, DVB-H, etc. Therefore the frequency planning or designing does not provide much help, because at almost any frequency, a system is being used, which accordingly faces interference.
Another known solution has generally been to increase RF shields such as RF encapsulations. However, due to, for example the increase of different communications systems, the physical space within the mobile phone is limited. RF shielding typically requires a lot of space. Therefore, RF shields are not a feasible solution for modern mobile phones. Furthermore RF shielding does not necessary prevent interferences within an integrated circuit (IC).
Generally, the interferences caused by the digital signals have been tried to be reduced by merely prolonging the rising and falling times of the pulses. Because the clock signal or the load of the circuit of the data bus is considered capacitive, interferences have been tried to be reduced by merely prolonging the rising and falling times and furthermore by limiting the circuit current. Known circuits of FIGS. 1 and 2 depict such solutions. In FIG. 1 capacitors Cx1 and Cx2, a current generator in a block B1, a bias stage and a controller stage prolong the rising and falling times of the pulses. FIG. 2 solution uses a feedback circuit, which is based on a capacitor, wherein the circuit establishes a kind of integrator or an integrating circuit. A pulse or signal V1 inputted to the circuit is show in FIGS. 1 and 2. Both circuits of the FIGS. 1 and 2 result in a pulse V2, which has sloping rising and falling edges (shown in FIG. 1 and 2), when the load is thus capacitive.
Yet another know solution resembles one of FIG. 1. However in this solution the rising and falling edges are formed from several parts, wherein there are different rising (and failing) speeds. An example of this kind of solution has been described in US patent publication U.S. Pat. No. 4,779,013.
However a common characteristic to all these known solutions is that generally they modify the edges of the pulses oblique and the upper corners of the edge of the pulse and lower corners of the edge of the pulse remain with sharp or pointed turnovers. Therefore the emergence of the interference is clear. The output of the circuit causes considerable sharp current peaks to the utilization stage. Therefore the interferences propagate to a large area of the circuit via supply lines and ground leads. These problems are not disadvantageous to, for example to desktop computers, because a computer does not typically have radio frequency components. Furthermore if the desktop computer has the radio frequency components, the radio frequency components can be shielded by RF encapsulation. However, the problem is considerable to portable hand held radio apparatuses such as mobile phones. Furthermore the problem is pertinent to laptop computers containing radio frequency components.
A further problem is that all the above solutions works reasonably well only if the load is capacitive (or resistive). However, the signal is typically always conveyed from circuit to another circuit within the circuit board through the stripline (alternatively referred to as microstrip). The stripline and the input capacitance establish troublesome impedance with respect to the output stage of the circuit (as show for example in FIG. 3). Therefore the above circuit solutions do not work as they should.
FIG. 4 depicts an ideal digital pulse (alternatively referred to as an ideal digital signal), wherein the output stage has a capacitance as a load. FIG. 5 depicts the same pulse of FIG. 4, which is coupled with a load having 50 mm long stripline and 3.5 pF capacitance. The pulse (or the signal) is distorted to near uselessness because the rising and falling edges, i.e. rising and falling moments, are indefinite. This is because the pulse (the signal or the like) reflects from the capacitive load at the other end of the stripline and adds to the signal coming from the output stage of the circuit, etc. Furthermore the signal causes considerable interferences to a large spectrum (as shown for example in FIG. 6). Yet furthermore the signal may interfere the reception and transmission channels of the mobile station and e.g. GPS reception.